How long can a unicellular organism live without nutrition? What happens after that? Does it depend on the domain?Why will they die? That's kind of difficult to answer. There are some hypotheses that unicellular organisms to a certain extent have multicellular characteristics. And this includes the ability to altruistically die for the benefit of the rest of the colony or species. By dying they would free up the limited resources, as well as allow their own proteins and cellular components to be taken up. Others believe it is just random, and some cells live a bit longer making them able to take advantage of what I said above, making the death in no way planned or organized.